Severance is matter of negotiation and contract. There are literally zero rights related to severance (unless there is a collective bargaining agreement) issues. So, no, there would not be any law that would protect you and allow you to get the benefits of vesting after the date determined in the severance agreement.
Your company is not negotiating with you, which they don't have to... anymore than you have to accept the agreement. Some employees have more negotiating leverage that others, where the company really wants them to sign the agreement to protect the company. Others, the company is basically offering the package as a good will gesture. If you are the kind of employee that the company will be hurt if you don't sign it, you can always reject the agreement.
But as it stands, sorry, there is nothing that would help you if you do accept the agreement.
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